1. Field of the Invention
The present invention relates to a high-frequency copper claded laminate (or called copper foil covered substrate) and a compound material used therein, wherein the substrate has a dielectric constant (Dk) less than 3.2 and a dissipation factor (Df) less than 0.005.
2. Description of Related Art
With the rapid growth of 3C (Customer Electronics, Computer, and Communication) industries, printed circuit boards (PCBs) have been correspondingly made with a frequency limit up to 1 GHz, from materials of low dielectric constants (Dk less than 3.2) and low dissipation factors (Df less than 0.005).
Conventionally, high-frequency substrates are mainly made of polybutadiene resins through the impregnating and high-temperature vacuum laminating processes traditionally used for making printed circuit boards (PCBs). However, polybutadiene has high tack, so the prepreg made therefrom is too tacky to be put into continuous manufacture of high-frequency substrates that traditionally uses automated layup processing.
Among the publicly accessible patent documents, U.S. Pat. No. 4,241,132 has disclosed insulating boards with a dielectric constant (Dk) between 2.2 and 20, which is made by impregnating fiber reinforcement with a polybutadiene polymer carrying a filler therein, and curing the polymer. However, this patent provides no solution to the problem that polybutadiene is too tacky to allow the insulating boards to be put into continuous automated manufacture.
U.S. Pat. No. 5,223,568 has provided a moldable thermosetting composition for making circuit boards. Therein, a polybutadiene or polyisoprene resin which is a liquid at room temperature and which has a molecular weight less than 5,000 is selected to be mixed with a solid butadiene- or isoprene-containing polymer (e.g., a thermoplastic elastomer). This patented scheme requires a high temperature cure condition at a high temperature (i.e., hot-pressing temperature greater than about 250° C.), yet leaves the problem that polybutadiene is too tacky to allow continuous automated manufacture unsolved.
For dealing with the problem related to the high tack of polybutadiene, U.S. Pat. No. 6,071,836 has proposed a compound circuit board material containing particulate filler in a large amount. The prepreg made therefrom is almost tack-free, so is suitable for the traditional automated layup processing. However, since the compound material has the filler taking more than 50 wt %, or up to 70 wt %, therein, it is nevertheless unfavorable to uniform impregnation to fabric reinforcement. In addition, the resultant circuit board is excessively rigid and tends to aggravate abrasion to drilling bits that drill the board in a drilling process, thus being adverse to processing.
In the field of plastic, polyphenylene ether resins are known as thermoplastic engineering plastics that endure high temperature and have excellent physical properties, such as low dielectric constant, low dissipation factor, high glass transition temperature, low water absorption, good resistance to high temperature and high electrical insulation, yet it needs to be modified because its poor melt flowability is against acceptable processability.
However, a polyphenylene ether resin having a number-average molecular weight (Mn) greater than 10,000 is less soluble in organic solvents and has high melt viscosity, making it unsuitable for circuit boards. On the other hand, a polyphenylene ether resin having a number-average molecular weight lower than 10,000 has its physical properties all lowered, including dielectric constant, dissipation factor, glass transition temperature and viscosity, to the extent that it fails to meet the need of a circuit board where high frequency, low dielectric constant and low dissipation factor are required.